1. Field of the Invention
The present invention relates to an optical fiber array including a substrate and a plurality of optical fibers positioned relative to the substrate and integrally bonded to the substrate with an adhesive agent, and an improvement in the construction of the substrate of the optical fiber array.
2. Discussion of Related Art
As well known in the art, an optical fiber array includes a plurality of optical fibers which are fixedly arranged in parallel with each other with a predetermined pitch. For example, the optical fiber array is connected to a planar lightwave circuit (PLC), or used for connecting two multi-core optical fiber cables.
Such an optical fiber array generally includes an substrate 3 for optical fiber array having a shoulder 9 formed at a longitudinally intermediate position thereof so as to define a thick portion on one side of the shoulder 9, as shown in FIGS. 1A and 1B. In this substrate 3 for optical fiber array, the thick portion serves as a grooved portion 3a having a plurality of V-shaped grooves 5 formed in its upper surface such that the V-shaped grooves 5 are arranged in parallel with each other, while the other thin portion is provided as a planar portion 3b having a flat upper surface.
Further, the planar portion 3b of the substrate 3 for optical fiber array supports a plurality of optical fibers 2 at their covered portions 2a, while the plurality of V-shaped grooves 5 accommodate the respective optical fibers 2 at their non-covered end portions 10 extending from the covered portions 2a, such that the optical fibers 2 are spaced from each other at a predetermined spacing pitch T and such that each optical fiber 2 is positioned at three points by two side surfaces of the corresponding V-shaped groove 5 and a retainer surface of a covering plate 4 disposed on the grooved portion 3a. In this condition, the plurality of optical fibers 2 are embedded in adhesive layers 6 (not shown in FIG. 1A) formed between the covering plate 4 and the grooves 5 and on the planar portion 3b, and are thus fixed to the covering plate 4 and the substrate 3 for optical fiber array, whereby the optical fiber array 1 is obtained. An example of this optical fiber array is disclosed in JP-2001-343547A. In an optical fiber array 1 shown in FIG. 2, the spacing pitch T of the optical fibers 2 is close to the diameter of each optical fiber 2, so that the optical fiber array 1 has a relatively high density of arrangements of the optical fibers 2.
To manufacture the optical fiber array 1 constructed as described above, an adhesive agent for forming the adhesive layers is injected into each groove 5 through a distal open end of the groove 5 located at the distal end of the substrate 3 for optical fiber array (which open end is remote from the shoulder 9), so that owing to capillarity, the injected adhesive agent fills a gap 7 left between the grooved portion 3a of the substrate 3 and the covering plate 4, and a gap 8 left between the optical fiber 2 accommodated in each groove 5 and the side surfaces of the groove 5. Where the upper gap 7 located above the optical fiber has a cross sectional surface area S1 which is smaller than a cross sectional surface area S2 of the lower gap 8 located below the optical fiber, a velocity at which the adhesive agent is introduced into the upper gap 7 is higher than a velocity at which the adhesive agent is introduced into the lower gap 8.
In the above-described case, the adhesive agent 11 introduced into the upper gap 7 reaches a rear open end of the groove at the rear end of the grooved portion 3a (on the side of the shoulder 9) before the adhesive agent 11 introduced into the lower gap 8, as shown in FIG. 3, so that an excess amount of the adhesive agent 11 initiates a drooping flow from the rear open end of the upper gap 7 before the adhesive agent 11 introduced into the lower gap 8 has reached the rear open end. As a result, the rear open end of the lower gap 8 is closed by a mass of the adhesive agent 11 which has drooped from the rear open end of the upper gap 7, so that a volume of air P is trapped in the lower gap, resulting in not only reduction in a force of bonding of the optical fiber 2 to the substrate 3, which gives rise to a risk of removal of the optical fiber 2 from the substrate 3, but also bending or straining of the optical fiber 2 due to thermal expansion and contraction of the air P, which causes an increase in optical transmission loss of the optical fiber.
Where the optical fibers 2 are fixed to the substrate 3 for optical fiber array, with their portions being embedded in the adhesive layers 6 formed on the planar portion 3b of the fiber substrate 3 in the conventional optical fiber array 1, the optical fibers 2 may be subject to stress concentration by the adhesive layers 6 at the shoulder 9 located between the grooved portion 3a and the planar portion 3b of the substrate 3, or may be damaged or broken in the presence of the shoulder 9, that is, in contact with a rear edge 12 between the grooved portion 3a and the planar portion 3b. 
JP-2000-275478A discloses an optical fiber array constructed so as to solve the problems discussed above. Namely, the grooved portion 3a of the optical fiber array disclosed in this publication has an upwardly convex rear end part formed adjacent to the shoulder 9, so that a crest 5a of each of two adjacent two walls defining each V-shaped groove 5 is rounded to have a relatively smooth curvature, in the reaar end part of the V-shaped groove 5 close to the shoulder 9, as shown in FIG. 4.
In the optical fiber array 1 constructed as described above, the stress acting on each optical fiber 2 due to the adhesive layers 6 gradually increases in a direction toward the shoulder 9, and the stress concentration at the rear edge 12 is prevented owing to a small amount of shift of a point of contact of the optical fiber 2 with the rear edge 12 toward the front end of the optical fiber 2, that is, owing to a shift of the stress point toward the front end of the optical fiber 2. Further, a length of contact of each optical fiber 2 with the rear edge 12 of the V-shaped groove 5 is increased owing to the curvature in the rear end part of the V-shaped groove 5, so that the stress acting on the optical fiber 2 is dispersed.
Although the conventional optical fiber array 1 described above is constructed to reduce the stress concentration at the shoulder 9 at the rear end of the grooved portion 3a, the contact of each optical fiber 2 with the rear edge 12 is inevitable, since angle θ of each V-shaped groove 5 defined by its two side surfaces and a depth h of the groove 5, which are indicated in FIG. 1B, are held constant over the entire length of the groove 5. Accordingly, the optical fiber array 1 has a risk of damaging of the optical fibers 2 at the rear edge 12 during assembling of the optical fiber array 1, or due to displacement of the optical fibers 2 upon a temperature change because of a difference in coefficient of thermal expansion between the optical fibers 2 and the adhesive layers 6.